Oil-in-water emulsions generally comprise an aqueous phase having suspended therein discrete oil droplets (particles) surrounded by a layer of at least one water soluble surfactant. Emulsion stability is largely determined by particle size; oil-in-water-emulsions having particle sizes that exceed 1 μm in diameter tend to be less stable and undergo creaming, coagulation and phase separation upon storage. Therefore, for most applications it is desirable to reduce particle size which generally results in significant increases in aqueous phase surfactant concentration. The smaller the particle size, the greater the combined particle surface area resulting in a need for more surfactant in the aqueous phase, thus more free surfactant in solution.
Oil-in-water emulsions are useful for treating ophthalmic conditions such as dry eye. Dry eye results from evaporation of naturally occurring water from the eye surface. Oil-in-water emulsions restore the eye's natural aqueous layer and provide an oil layer over the newly added aqueous layer to prevent further evaporation. For maximum efficacy the oil must be spread evenly and freely over the eye surface and the solution must be stable on storing to permit frequent application to the eye. However, stable oil-in-water emulsions can contain excessive amounts of free surfactant. The free surfactant can wash away the tear film's natural lipid component and damage the mucin layer covering the cornea or conjunctiva thus exacerbating dry eye. Therefore, stable oil-in-water emulsions having an average particle size less than 1 μm in diameter that contain non-Irritating amounts of eye-damaging free surfactant in the aqueous phase are desirable.